Absolute muscle strength is one goal of a strength-training program, of course. Other training goals are also important, such as endurance, flexibility, and speed. In particular, development of explosive strength, or jumping ability, is important for all sports. This ability is not well developed with conventional peak-contraction training, which keeps continuous tension on the fully contracted muscle. Another training goal, especially for sports, is to overcome the so-called bilateral deficit. The bilateral deficit is the difference between the maximal effort that can be developed with one arm (or leg) and two arms (or legs) working together. For example, persons generally cannot exert a force during a two-arm effort that is twice that of the one-arm maximal effort.
Plyometric training is the field of strength training intended to increase the reversal strength of subjects. Plyometric training achieves its effects by taking advantage of the energy stored in contracted muscle. Such training can also be used to train individuals to overcome the bilateral deficit.
A muscle develops its maximum force during a rapid eccentric contraction. An “eccentric contraction” occurs when a muscle is forced to lengthen due to the high external load, although it may be fully activated. “Contraction” in this context does not necessarily imply shortening. Contractions that permit the muscle to shorten are referred to as “concentric contractions.” In concentric contractions, the force generated by the muscle is always less than the muscle's maximum. Of course, muscles seldom perform one type of contraction in isolation during athletic movements.
If a muscle is stretched, energy is stored by the elastic components of the muscle. This stored energy is available to the muscle only during a later contraction. When a concentric contraction occurs immediately following an eccentric contraction, then the force generated can be dramatically increased. This increase in force is lost if the eccentric contraction is not followed immediately by a concentric effort. To express this greater force the muscle must contract within the shortest time possible. This whole process is frequently called the stretch shortening cycle and is the underlying mechanism of plyometric training. Plyometric training places increased stretch loads on the working muscles. As the muscles become more tolerant to the increased loads, the stretch-shortening cycle becomes more efficient.
Prior-art training machines are primarily devoted to developing high strength through the peak-contraction principle. There is a need for a training apparatus that will allow athletes who are already strong to develop reversal strength, especially through a large range of motion.